1. Field of the Invention
The present invention relates generally to a hydraulic control device for a hydraulically operated clutch device for an automotive vehicle, and more particularly to a technique for preventing or minimizing drag due to partial frictional engagement between driving and driven rotary members of a friction clutch which is provided with an automatic wear compensating device for improved engagement response.
2. Discussion of the Related Art
There is known a hydraulically operated friction clutch which has a driving rotary member, a driven rotary member, a movable member movable in the axial direction of the rotary members, an an actuator piston axially slidably received within a cylinder bore, for moving the movable member to effect frictional engagement of the driving and driven rotary members. The actuator piston is exposed to a pressure chamber formed in the cylinder bore, so that the actuator piston is moved by a pressurized fluid supplied to the pressure chamber, where the movable member is axially moved to frictionally couple the driving and driven rotary members, to transmit a rotary motion of the driving member to the driven member. In this type of hydraulically operated friction clutch, the driving and driven rotary members are frictionally coupled to each other via a friction member, which is subject to progressive wear during use of the clutch. Therefore, the distance of movement of the piston required to commence frictional engagement of the driving and driven rotary members is inevitably increased. This means a progressive increase in the amount of flow of the working fluid supplied into the pressure chamber, resulting in an increased amount of time necessary to complete the frictional engagement of the clutch, or a longer response time of the clutch, upon starting of the vehicle or during a shifting operation of the transmission. To solve this drawback, there has been proposed an arrangement as described in Laid-Open Publication No. 60-23660, wherein the engagement starting position of the clutch which varies with the amount of wear of the friction member is detected and stored in a memory, so that the working fluid is supplied to the pressure chamber at a comparatively high rate to move the actuator piston to the engagement start position at a high speed, and the fluid supply rate is lowered after the piston has reached the engagement starting position. However, this arrangement requires a complicated control system.
In the meantime, the present inventors developed an improved friction clutch as disclosed in U.S. Patent Application filed Apr. 3, 1987, Ser. No. 033,616, claiming the priority of the corresponding Japanese Patent Application No. 61-78845 filed Apr. 4, 1986. This friction clutch has a driving rotary member, a driven rotary member, an actuator piston operable for frictional engagement between the driving and driven rotary members, a pressure chamber supplied with a controlled pressurized fluid to activate the actuator piston, and a proximity shut-off valve disposed in the actuator piston, for permitting the pressurized fluid to be discharged from the pressure chamber through a discharge passage when the actuator piston has reached a ready-to-operate position from which the actuator piston is moved to start the frictional engagement between the driving and driven rotary members. According to this arrangement, the movement of the actuator piston to frictionally couple the driving and driven rotary members is stopped at its ready-to-operate position of the piston, or the engagement start position is shifted according to the amount of wear of the clutch, to assure an improved response of the clutch irrespective of the amount of wear of the friction member. To effect the frictional engagement of the clutch, the discharge passage is closed to increase the pressure in the pressure chamber, thereby moving the actuator piston to frictionally couple the driving and driven rotary members.
However, the hydraulically operated friction clutch proposed in the above-identified U.S. Patent Application is not completely satisfactory. Described more specifically, the discharge passage connected to the pressure chamber by the shut-off valve usually includes a fluid passage formed so as to originate from the pressure chamber, through the housing of the clutch and/or the housing of the transmission coupled to the clutch, and a suitable conduit connected at its one end to the above fluid passage and at the other end to an oil reservoir under the atmospheric pressure. Since the discharge passage has a resistance to a flow of the fluid therethrough to the oil reservoir, there exists a pressure difference between the pressure chamber in the clutch and the oil reservoir even after the shut-off valve is opened to permit the pressurized fluid in the pressure chamber to be discharged to the oil reservoir. In other words, the pressure in the pressure chamber acting on the actuator piston is maintained at a comparatively high level above the atmospheric pressure even after the shut-off valve is opened. In this condition, therefore, the actuator piston is advanced from its ready-to-operate position in a direction to effect the frictional coupling of the clutch, since the pressure in the pressure chamber acting on one end of the actuator piston is higher than a substantially atmospheric pressure which acts on the other end of the piston. Consequently, the friction clutch suffers from drag due to partial frictional engagement of the driving and driven rotary members, even while the clutch is not commanded to be engaged. This drag or partial engagement of the clutch results in shortening the life expectancy of the clutch, and leads to unstable idling of the engine, or requires an increased fuel supply to the engine or assure stable idle running of the engine. Thus, the clutch arrangement previously proposed by the present inventors suffers from relatively low durability, and leads to relatively low fuel economy of the engine.